Hi,
When using a third party CC processor, how would you go about billing clients if they use extra bandwidth etc. From what I understand, you do not have access to customers credit card numbers, therefore is there somewhere that you can login to make extra charges if needed.
Any info on this topic would be greatly appreciated. (I will probably use Worldpay)

Typically, yes this is a problem. Other than asking the processor to up the charge for that particular transaction, or sending them another order link (either to upgrade, or a one time link), there isn't much you can do.